This invention relates generally to the surface mounting of integrated circuits (ICs), and more particularly, to a die feeder device that performs the function of removing each IC, or die, from a sawed, expanded wafer disposed on a flexible adhesive film, placing the die on a conveyer, and conveying the die to a pick-up location.
In the mass production of printed circuit boards having a plurality of ICs surface-mounted thereon, automated machinery accept die from "chip feeders" and then mount them at a predetermined location on the circuit board. The chip feeders have previously taken the form of tape and reel-type feeders which remove the die from a plastic tape and present them to the automated assembly machinery. An example of a reel type tape feeder is shown in U.S. Pat. No. 4,437,232, issued to Araki et al. on Mar. 20, 1984. The advantage of the tape feeders is that they take up a small amount of space during the assembly of the circuit board, thus a large number of tape feeders can be placed adjacent to one another and utilized by a single assembly machine. Tape feeders are also capable of feeding die at a very rapid pace. The present invention takes the place of these "tape feeders" and obviates the steps of placing die onto the plastic tape then removing them from the plastic tape. The present invention is capable of performing this function of presenting die serially directly from a wafer without taking up significantly more space than a tape feeder. By reducing the handling of bare die on its journey from wafer to assembly, significant expense reduction is realized, and the opportunity for damage to individual die is reduced, which improves the reliability of the end product.
Because the present invention is intended to replace current tape feeders, it is designed to maintain the narrow form-factor width standard of 80 mm which is common in the industry. The narrow width enables many die feeders of the present invention to be positioned side by side down the length of an assembly conveyor which carries printed circuit boards from station to station while various electrical components are installed on them without taking up too much floor space. Maintaining this narrow profile required many design innovations herein disclosed.
The present invention picks die from an expanded wafer and presents them, via a conveyor, to an automated assembly machine. The use of an expanded wafer provides important advantages when picking die. "Expanded wafer" means a sawed wafer placed on a flexible adhesive film which is then stretched so that individual die can be spaced from one another. The film is then maintained in this stretched condition by placing it between two rings clamped together. The gap between adjacent dies permits reliable pickup and prevents "chip-out" (damage to the picked die or adjacent die caused by scraping the die edges during pick-up). In addition, a machine vision system is employed by the present invention to perform die recognition using the die edges, which requires gaps between the die, permitting them to be perceived by a visual imaging apparatus.
U.S. Pat. No. 4,876,791 to Michaud et al. discloses an apparatus for removing die from an expanded wafer and placing the die on a substrate. This patent teaches holding the expanded wafer horizontally with the wafer side facing down. A pick and place head is positioned beneath the wafer and above the horizontally-disposed substrate. This configuration permits the pick and place head to remove the die from the wafer and place it on the substrate with minimal movement. However, it suffers from the disadvantage that it is not compatible with existing assembly machinery and requires a significant amount of horizontal real estate, which becomes especially problematic when a plurality of chips are to be placed on each substrate. In this case the substrate must make a separate stop beneath each wafer and a separate mounting assembly must be utilized for each chip to be placed on the substrate.
Although direct die feeders have been known in the prior art, industry has not accepted them. Presumably, this is so because they lack specific features required by the industry, such as the high speed and small footprint currently made possible by the tape feeders.
U.S. Pat. No. 5,671,530 to Combs et al. discloses an apparatus which removes die from an expanded wafer oriented vertically for use in the environment intended for the present invention. The patent to Combs et al., however, suffers from the disadvantages of only being capable of handling one die at a time using pick and place head assembly 104 shown in FIG. 6 of the Combs et al. patent. Pick and place head assembly 104 travels from a pickup location 96 to a transfer location 106. Head assembly 104 cannot return to the pickup location 96 until the circuit board assembly machinery takes the die at transfer location 106. This pick-move-transfer-move-pick cycle is extremely time-inefficient, making the system impractical for many customers who mass produce circuit boards. It is important to understand that the conveyor shown in the Combs et al. patent operates to transport printed circuit boards during assembly, not to transport bare die to a pickup location as does the conveyor of the current invention.
The Combs et al. patent also suffers from the disadvantage of only being able to present chips in a "flipped" (circuit-down) orientation. There are currently two primary methods of surface-mounting chips onto a substrate or circuit board. The conventional wire-bond or tape-bond methods includes placing the chip on the substrate in the conventional, circuit-up orientation, and electrical connections being made from electrical contacts on the top of the chip to contacts on the substrate using metal wires or tape. The flip-chip method includes forming solder bumps on the art side of the die, then placing the die, circuit-down on the substrate, aligning the solder bumps on the die with metal contacts on the substrate and holding the die in position while electrical and mechanical connection is made by reflowing the solder bumps. The Combs et al. invention is limited by only being capable of presenting chips in the flipped orientation.